Fix indefinite hang on invalid PKCE client_id

[AtomikkuLabs]

Fixes indefinite hang when an invalid client_id is put into SpotifyPKCE and get_access_token is called. A preflight has been made to test out the client_id beforehand. Not sure if this is the most valid approach but it seems to work and gives a clear error.

PKCE issue was noted by me here

[dieser-niko]

Sorry for the late reply. But I have to say, I really like it!

Just one thing. We won't be able to verify the redirect_uri, are we?

[AtomikkuLabs]

Sorry for the late reply. But I have to say, I really like it!

Just one thing. We won't be able to verify the redirect_uri, are we?

Thanks for the reply @dieser-niko. An invalid redirect_uri will cause the preflight to fail as well but that does not matter as we only raise an invalid client_id error and prevent the webpage from opening (therefore freezing the program) when we get a 'invalid_client' error from the preflights response. If the redirect_uri is wrong it will produce a generic 'invalid_grant' response which will cause it to just continue and open the webpage as normal, producing "INVALID_CLIENT: Invalid redirect URI" and freezing the program. I do not see this as an issue as developers should be putting a correct redirect_uri in statically. Since programs like my own need to allow a user to input their own client_id (in which a user may input garbage), it needs to not freeze the program like it currently does without my preflight.